System for reproducing sound from a sound record



July 23, 1935. J. H. HAMMOND, JR 2,008,824

' SYSTEM FOR REPRODUCING SOUND FROM A SOUND RECORD Original Filed Aug.21, 1951 5 Sheets-Sheet ,,1

1N VENTOR .4 TTORNEY8 J y 1935. J. H. HAMMOND, JR 2,008,824

SYSTEM FOR REPRODUCING SOUND FROM A SOUND RECORD Original Filed Aug. 21,1931 3 Sheets-Sheet 2 Tazalaae tabatk Con Ira/(Illa nw fs Due *0 firstC0 mfrof (3/10/77? e/ Inpai 8/ 270] Decrease ofB/as Due toSfgfia/ReSi/f/cation INVENTOI}? A TTORNE Y5 O N) I (D 11:

U [NVENTOR July 23, 1935. J. H. HAMMOND, JR 2,008,824

SYSTEM FOR REPRODUCING SOUND FROM A SOUND RECORD Origi nal Filed Aug.21, 1951 3 Sheets-Sheet 5 A TTORNEY$ Patented July 2 3, 1935 L I ESTATES assert SYSTEMTFOR REPRODUCING SOUND FROM A SOUND EECOED John HaysHammond, .ln, Glcucesten'Mass;

Application August 21, 1931, een No. 553,502

'Renewed September 14, 1933 30 Claims.

This invention relates to the production of sound from sound records,and more particularly to an improved method and system for controllingthe amount cf-sound in accordance with "-5 the characteristics of theenergy derived from therecord.

The invention provides for controlling the sound production so thatundesired sounds, such as those due to ground noise and needle scratchare not produced unlessaccornpanied by desired sounds such as those dueto speech and music, which mask the presence of theundesired sounds.

The system comprises a signal amplifier which :receives energy from asound record and applies the same after amplification to a suitablesound 'producing'device, such as aloud speaker. A control device is alsoutilized which is actuated in accordance with the original record andcontrols the amplification characteristics of the signal amplifier. Bythis means a variable amplification characteristic is obtained by whichstrong signals are amplified to a greater extent than weak signals. Thepresent invention relates particularly to the manner in'which thecontrol is exercised, and achieves new and useful results as regardssmoothness of operation, speed of action,

the degree of perfection in elimination and mask ing of undesiredsounds, and in other respects.

The characteristics of the control device are such that weal: signalcurrents outside theground noise and scratch frequency range cause arelatively great change in the amplification of the signal amplifier,while weak currents in the ground noise and scratch frequency rangeproduce little or no change; For stronger currents the control is notrelatively so great nor is there so great distinction as to whatfrequencies may exercisethe control. Initialoperating conditions of thesignal amplifier are such that it passes practically no currentwhatsoever. Because of the discrimination against scratch and groundnoise currents by the control device,these currents do notrender thesignal amplifier operative, but the desired signals render the amplifieroperative and cause the same to amplify the currents produced b'ythesound record by a variable amount which is determined by the frequencyand amplitude characteristics of said currents.

For this purpose the control device includes two or more controlchannels which are adapted to exert different influences on thesignalamplifier. In particular two such control channels may be employed, thefirst capable of producing relatively great change of signal amplifierperformance, and the second capable of producing basis.

relatively small change. The first and potentially stronger channel,however, is arranged with frequency characteristics to discriminateagainst control of the signal amplifier by currents of scratch or groundnoise frequencies, and further is arranged so that its control action onstrong signals is limited. Thesecond or relatively weaker controlchannel, which does not necessarily discriminate against currents ofscratch or ground noise frequencies, doesnot contribute appreciably tothe control on Weak signals, but predominates over the first channelafter limiting action has prevented theifirst channel from exercisingfurther control.

By suitable adjustment of the frequency discrimination characteristicsof the potentially stronger control channel, and by suitably limitingthe actionof each potentially stronger channel prior to any limitingaction on a weaker channel, a suitable control arrangement of the signalamplifier is produced which is automatically variable both on afrequency basis and signal strength As a result,-high frequency currentsof the scratch and ground noise frequency range occurring in the inputto the signal amplifier produce littleor no outputsound energy unlessthey include currents due to relatively strong desired signal ;tones, orare accompanied by lower frequency energy, therpresence of either ofwhich in the sound output masks the presence of the undesired noises.

Although the novel features which are characteristic of. this inventionwill be particularly pointed out in the claims appended hereto, the

finvention itself as to its objects and advantages,

the mode of itsoperation, and the manner of its organization may bebetter understood by referringcto the following description taken inconnection'with' the accompanying drawings forming a part thereof, inwhich Fig. 1 illustrates diagrammatically the system as applied to thereproduction of a sound record -or a talking moving picture projector ora phono- -venience, but they are intended to be as generic in theirapplication to similar partsas the art will permit.

Like reference characters denote like parts in the several figures ofthe drawings.

Referring to the accompanying drawings and particularly to Fig. 1, asou-roe of electrical energy is provided at the primary of transformer2|, from sound on film or sound on disc in accordance with the settingof a switch ll. For producing elec-- trical energy from sound on disc,there is provided a phonograph pickup device l8. For producingelectrical energy from sound on film, there is provided a light sourceID, from which light traverses lens and after passing through apertureI? of aperture plate |3 impinges upon the moving sound track of film l4.The light transmitted through the sound track and varied in accordancewith the record thereon, impinges on a photo-electric cell |5, which isconnected in circuit with a battery and the primary of transformer IS.The secondary of transformer I6 is connected to two terminals of switchI1, and the pickup I8 is connected to two other terminals of said switchin such a manner that energy from either source may be connected to theprimary of transformer 2 I. The parts so far described are typical ofany source of electrical tone input to the transformer 2|.

The signal amplifier comprises a first stage of amplification includingtube 22 which is coupled by a transformer to a second stage ofamplification including tubes 53 connected in push-pull relationship.The output of said tubes 53 is coupled by transformer 54 to a loudspeaking device 55, for example of the dynamic type having a field coil56. A resistance 52 is connected across the primary of transformer 5|and serves as an equalizing impedance.

The output of transformer 2| is connected to the input circuit of tube22, the grid return including resistances 23 and 24, battery 43 andpotentiometer 43a. There is accordingly provided a grid bias composed ofthe fixed bias due to battery 43; a manually adjusted bias due topctentiometer 43a and an automatically variable bias due to thepotential drop caused by currents passing through resistances 23 and 24.

The complete control channel includes an amplifier 21 to which energy isapplied from the secondary of transformer 2| through coupling condenser2 la. The output circuit of said amplifier includes a transformer 34 bywhich energy is transferred to a rectifier circuit including rectifier35 and resistances 36 and 23. The control channel also includes anamplifier 28 which is coupled to the output circuit of amplifier 21through coupling condenser 28a and has included in its output circuit'atransformer 31 which transfers energy through a filter network 38 andtransformer 35 to a rectifier circuit including rectifier 4| andresistances 42 and 24. The filter network is preferably so designed asto discriminate against high frequency tones, such as the undesirabletones, above-mentioned.

A limiting device including tube 48 is connected across resistance 24and is adjustable by means of adjustable tap 45 on potentiometer 44which is connected across battery 43, the adjustment being such as tolimit the voltage drop across resistance 24 to a predetermined value. Alimiting device such as tube 41 is also connected across resistances 23and 24 and is controlled by adjustable tap 45 of potentiometer 44 tolimit the voltage drop in said resistance and to prevent the bias changefrom exceeding a predetermined value.

All parts of the complete control channel which contribute indetermining the current through resistor 24 constitute the first orpotentially stronger control channel, and all which contribute indetermining the current through resistor 23 constitute the second orweaker control channel. Although some parts of the complete controlchannel are for simplicity common to both, such as amplifier 21, it isobvious that the two individual channels are substantially independentor could be made so to any desired degree.

In the operation of Fig. 1, battery 43 is so chosen and potentiometer43a is so regulated that in the absence of signal, little or no platecurrent passes from the plate battery to the plate of tube 22. Underthese conditions, the internal impedance of tube 22 is very high and forvery weak signals the amplification of the stage including the device 22is small or actually much less than unity so that the voltage to thegrids of push-pull amplifier 53 may be even negligible compared with thevoltage applied to the grid of device 22. In the absence of signal theremay be a small current through resistances 23 and 24 due to energy fromthe filament heating devices of rectifier tubes 35 and 4|. This initialcurrent may be compensated for by the potentiometer adjustmentabove-mentioned.

When a signal is impressed upon device 22 a corresponding signal willsimultaneously be impressed upon the control channel including devices21 and 28. That part of the amplified energy output of device 21 whichpasses through transformer 34 is rectified by device 35 and therectified output is smoothed out and delivered to resistance 23 causinga current to flow in the direction of the arrow. A small portion of theoutput energy of device 21 actuates device 28, the output of whichtraverses filter network 38 is rectified by device 4| and produces acurrent which flows in the direction of the arrow in the resistance 24.Consequently, the bias of tube 22 is decreased in accordance with thesum of the voltage drops in resistances 23 and 24, thereby lowering theinternal impedance of the tube and permitting it to become moreconductive.

By reason of the variable internal impedance of device 22, it isdesirable to provide an equalizer impedance 52 across the input oftransformer 5|. This impedance serves to maintain the external outputimpedance of tube 22 substantially independent of the frequency, and theimpedance of the devices connected to the primary of transformer 5|substantially independent of the internal impedance of the tube,

whereby the fidelity of amplification is substantially the sameregardless of the signal strength. Although a resistor type equalizer 52has been shown for purposes of illustration, it is obvious that animpedance network of any desired type may be employed.

For weak input signals, the control on the per formance of tube 22 islargely exercised by rectifier 4| which is supplied by transformer 31,since the output of tube 28 exceeds that of tube 21. Means is provided,however, for limiting the voltage drop across resistance 24 so that forhigher signal strengths, the control channel including said resistanceis prevented from exerting as much influence as the control channelineluding resistance 23.

For this purpose a limiter 48 is provided which operates to limit theoutput of rectifier 4| when the output voltage across resistance 24exceeds a predetermined value. In the absence of ourrent throughresistance 24, the plate of limiter 48 is negative with respect to thefilament due to the bias impressed into the circuit by the setting ofcontact on potentiometer 44. Device #38 is accordingly non-conductiveuntil the voltage drop across resistance 24 exceeds the bias voltageabove-mentioned, after which the device 53 becomes conductive andprevents further voltage change across said resistance.

Resistance 32 assists in the limiting action and also operates, as apart of the filter network for the output of rectifier ll;

The limiters? operates'in accordance with the combined voltage dropacross resistances 23'and 2 5 and is adjusted by means of contact lfi ofpotentiometer it to limit the voltage drop across said resistances inthe manner similar to that pointed out in connection with limiter' lfi.Limiter ll is preferably adjusted so that the combined voltage: dropacross resistances 23 and 24 will not reduce the grid bias of tube 22below the value for which maximum undistorted signals may be obtainedtherefrom.

In general, the limiter 48 isadjusted to becomeoperative at less signalstrength than limiter ll, so that the relatively weaker control channelbecomes the relatively stronger channel on strong signals. 7

,It is seen then that by the use of two control channels, and a limiteron the potentially stronger channel, a quick action upon increase ofsignal is produced upon the amplifier control, with gradual taperedaction as strong signals are used. The quick action at Weak signalsrefers to rapid change of signal amplifier'perionnance with change ofinput signal, by which small signals may change the signal amplifierfrom a non-operative to an operative condition. The quick action alsorefers to the rapidity with which the change of operation is broughtabout, due to the large mount of energy availableiol making the change.In both senses, the quick action is a desirable: feature of theinvention.

The action of the two control channels is depicted in Fig. 2, in whichdecrease of bias of the tube 22 due to the presence of the rectifierelements ll and 35, is shown as dependent upon the input signal for asingle frequency, say in the middle or" the audible range. Curve A?depicts the change of bias due to the first or potentially strongercontrol channel including the second stage amplifier device 28,rectifier 3 l, and limiter 48. Curve B depicts the change due to thesecond or relatively weaker control channel including the first stageamplifier device 2'5, rectifier 35 and limiter Al. Curve .C depicts thetotal change of bias due to both control channels, and is seen to becharacterized by a sharp decrease of bias for small signals. withgradual tapering effect, and finally no" further decrease of bias athighest signals. V

For a high frequency signal input, with the potentially stronger channelnot operative :due

to action of filter 38, there would be no contri bution to the change ofbias due to curve A, and curve B would coincide with C in depicting thetotal change of bias due to the entire control channel. For othervirequencies'ior which filter 38 is only partially effective, the controlcurve would lie between curves C. and B as here shown. It is to beunderstood that the nature of the control curves for differentfrequencies may be modified byadjustments of relative inputs to: thefirst and second control amplifiers, by adjustment of the limiters, andby adjustments "of frequency characteristics of the twoicontrol channelsso that 'a great variety of relations between gain ratio of the signalamplifienand the wave form characteristics or the signals maybeattained.

Onesuch arrangement, for example, would proportion curvefiffC such thatsubstantially even decibelfexpansion would beobtmied .for all use ful'signal :input values, with no frequency discrimination by the channelsfOr another arrangement, for example, by making the weaker channelslightly more responsive to the high frequencies, as by adjustment'ofcondenser 22?, and of the characteristics of transformer 34 etc, wouldcause substantially equal output of the entireaniplifier forallirequencies on high signal input, although the high frequencies arediscriminated against on weak input. Or a third arrangement, asby-removal of the limitertube laor renderingit ineffective by adjustmentof contact 46 would .causeathe signal amplifier to perform substantiallyas a normal amplifier with expansion only on very weak signals, andnormal performance over a very. Wide range ofv signalinputs. Thislast-mentioned arrangement is useful for sound inputs with fairly markeddifference of level between input straynoises, and lowest desiredsignal, for which complete ground noise elimination is desired withoutdynamic expansion of signal currents. All these arrangements are to: beconsidered within the scope of the invention, or of structuralmodifications thereof.

.A modification of they invention as applied in the control of aplurality of stages of a signal amplifier is shown in Fig. 3, in whichtheinput may be from the terminals of switch ll, and the output circuit'59 may be connected to a loudspeaker, a telephone line,.or a final highpower stage. (not shown). Across the input is a potentiometer 63, havingadjustable taps GI and 62 :for controlling the distribution of energy toin- .put transformers '2! of the signal amplifier and H of the controlamplifier. The signal amplifier includes tube 22, couplingtransformerbl; push-pull amplifier tubes 53 and an output trans former54, all similar to those described in con nection with Fig. 1, exceptfor the use of separate heater tubes.

.Across the primary of transformer 5! is a re sistance '63, and betweenplate and ground is a condenser es in series with a, variable resistance55.

the frequency transmission characteristics of the signal amplifier maybeadjusted. Equalizer resistances are also provided for the windings oftransformer 5d. Thergrid andplate returns are bypassed -to-ground bysuitable condensers, care being :taken that. the grid return bypasscondensers are not or excessive capacity. The cathodes of tubes 53 arejoined together, and grounded through an indicating milliammeter 6?. Thecathode of tube 22 is bypassed-to ground. The operating platepotentialsfor the tubes, a potential for the cathode of tube 22, andheater currents are supplied inthe usual manner from substantially fixedsources to be described later. The operating biasfor the tubes, however,may be substantially fixed for normal amplifier operation,

or may be made automatically variable. For this purpose. the grid returnof tube?! is connected to one centerterrninal-GQ of a double pole doublethrow switch 68; and-the common grid return of This network constitutesan impedance equalizer circuit, and also a tone control by which tubes53 is connected to the-other center terminal Hi. When this switch isthrown to right as shown in Fig. 3, the signal amplifier performs as anormal amplifier, with suitable fixed bias values. But when the switchis thrown to the left, the signal amplifier performs as a controlledamplifier with grid bias supplied from the control circuits. The controlchannel, operating from the input transformer includes first stageamplifier 12 with its output transformer 34, and second stage amplifierl3 with its filter 38 and output transformer 35. One side of thesecondaries of the transformers 34 and 39 are connected to anodes ofrectifiers and M respectively, while the other sides are joinedtogether, and to conductor 14, which is at substantially fixed potentialduring operation, and is bypassed to ground. In addition, voltage fromthe secondary of transformer 34 is fed to the grid of the second stageamplifier 13 by the use of coupling condenser and resistance 16.Suitable operating potentials for the control amplifiers are supplied inthe usual manner; in the present case resistances 'I'l' and 18 beingused to out down the plate supply voltage. The outputs of rectifiers 35and 4| are fed toa filtering, combining and distributing controlnetwork, including resistances 88, 8|, 82, 83, 84, and a plurality ofcondensers. The output of rectifier 4| flows through resistances 83 and84 to conductor 14, while the output of rectifier 35 flows throughresistors 88, 8|, 82 and B l. Opposite sides of resistance 8| areconnected to terminals 85 and 86 of switch 68, so that with switch 68thrown to the left, the change of bias on tubes 53 due to rectifiers 35and 4| is the voltage drop in resistances 82 and 84, while the change ofbias on tube 22 is the drop due to the drop in resistances 8|, 82 and84. An adjustable condenser bank 81 is provided for adjusting the timingto make it slower, if desired than that provided by the variouscondensers of the grid return circuits. Limiter tubes 48 and 4? areprovided to control the rectifier output, the anode of tube 48 beingconnected to the junction of resistances 82 and 84, and the anode oftube 41 being connected to terminal 85 of the switch 63.

The operating power for the tubes of the entire circuit is supplied frompower transformer 88, operating from an alternating current line. Thesecondary winding 85 supplies heater current to all tubes, it beingunderstood that there is no current flow between heater and cathodes ofthe tubes in their various functionings. Windings 90 and 8| supply arectifier system of usual design, comprising rectifier tubes 92, andfilter 93. The center tap of filament winding 89 is connected to thenegative output side of the filter. Plate operating potential for alltubes is supplied from the positive side of the filter. In order toprovide a ballast load on the rectifiers so that the output voltage willnot vary excessively upon change of plate current of amplifier tubes 22and 53, and to provide various fixed and variable biases, a resistorassembly is provided including fixed section 94, variable section 85,and tapped potentiometer section 95. In practice, section 84 may be thefield winding of a loud speaker, as also may be one of more of thefilter choke windings. Variable resistance section 85 may be used toadjust the system upon changing tubes, and for average tubes, section 85may be adjusted to approximately half its maximum value. The tappedsection is provided with a variable tap 9'! connected to the conductor14, for determining the bias on the amplifier tubes as dynamicmultipliers with very weak or no signal; variable taps 98 and 99 fordetermining the voltage at which tubes 48 and 41 commence to operate aslimiters; fixed taps I88 and lfll for determining the fixed bias fortubes 53 and 22 for normal amplification, with switch .68 to the right;tap I02 for biasing the control amplifiers 12 and 13; tap I83 for groundand bias reference point and bias for cathodes of all amplifier tubesexcept 22; and tap H34 at the junction of resistances 98 and 95, forproducing a slightly positive bias on cathode of tube 22.

In the operation of Fig. 3, after proper settings have been made, theincoming signals are suitably impressed upon the signal amplifier 22 bytransformer 2|, and upon the control amplifiers 12 and 13 by transformer1|. Rectifiers 35 and 4| are operated ina manner similar to those inFig. 1, except that rectification by the stronger channel takes greaterprecedence over rectification by the relatively weaker channel, sincetransformer 38 delivers more power than transformer 34. Thus, therectified output of device M delivers current to resistance 84, andplaces a positive bias upon the cathode of rectifier 35, so that saidrectifier cannot rectify until the action of device 44 has been limitedby tube 48, and the inputsignals are sufficiently great to overcome thelimited bias due to current flowing through resistance 84. This resultsin improved action, first by rendering rectifier 35 inoperative untiltube 4| has ceased producing further effect, and second by permittingtransformer 34 to deliver a very high voltage to the second stage tube13, for quick starting of the control circuits. In this manner thesystem differs from that described in connection with Fig. 1 in whichsubstantially independent action of the two channels was provided. Itwill be understood that further modifications of the invention arereadily possible, such as by connecting the returnterminal of the secondary of transformer 34 not to the junction of resistances 82 and 84,as in the first case, nor to conductor 14, but at some other point offixed or variable potential, such as an adjustable tap on resistorassembly 96, or a tap on resistance 84.

As a result of the production of control currents in the resistancenetwork, the biases of tubes 22 and 53 are lowered in accordance withthe voltages developed therein, the gain ratios of the signal amplifierstages are increased, and the output volume expanded. In case excessivesignals are impressed upon the control channel, limiter 41 operates tohold the biases especially of tubes 53 to the value for which maximumamplification is possible without signal distortion.

Although the arrangements shown pertain to the control of one or morestages of signal amplification, by bias grid control for dynamicmultiplication or expansion, it is clear that the invention relates alsoto control for other purposes, of signal amplifiers for dynamic divisionor compression, and further is applicable for control of a signalamplifier by plate operating potential variation, control by additionalelectrodes such as in tetrodes, pentodes, etc, and any combinationsthereof, as well as control by electrodynamical methods includingautomatic control of a potentiometer or a rheostat.

Although only a few of the various forms in which this invention may beembodied have been shown herein, it is to be understood that theinvention is not limited to any specific construction, but might beembodied in various forms without departing from the spirit of theinvention or the scope of the appended claims.

What is claimed is:

1. A system for producing sound from a sound record comprising incombination with a trans duoer, a control network, means whereby thevoltage drop'in' said network controls the transmission characteristicsof said transducer'means for introducing currents into said network froma plurality of channels having difierent transmission characteristicsand means for actuating said channels in accordance with the source ofsignal current applied to said transducer.

2 'A- system for producing sound from a sound record comprising incombination with an amplifier', a control network, means whereby thecurrent in said network controls the amplification factor of saidamplifier, means for introducing currents into said network from aplurality" of channels having diiferent transmission characteristics andmeans for actuating said channels in accordance with the source ofsignal current applied-to said amplifier. r

3. A system for producing sound from a sound record comprising incombination with a'space discharge amplifier, a plurality ofresistancenetworksadapted to control the amplification characteristicsof said amplifier, individualchannels for actuating each of saidnetworks and means for actuating said channels in accordance with theinput signalto saidamplifier.

41 A system for producing sound from asound record comprising incombination with a space discharge amplifier, a plurality of resistancenetworks associated therewith,-means whereby the voltage drops in eachof said networks controls the amplification ratio of said amplifier andmeans for applying currents to said networks in accordance with theinput signal current to said amplifier, the currents applied to thevarious networks having different characteristics with respect to saidinput signal current."

5. A system for producing sound from a sound record comprising incombination with a space discharge'amplifier, a resistance network adapted tocontrol the amplification ratio of said ampli fierin accordancewith the potential droptherein, means for passing arectifiedcurrent'through said network including a pair of control channelsactuated in accordance with the input signal strength, one of saidchannelsbeing adapted to produce 'a greater voltage drop in saidnetworkthan the other of said channels for agiven input signal strength andmeans for limiting the control of said first channel whereby the controlexercised by said second channel predominates at high signal strength. I

6; A system for producing sound from a sound record comprising incombination with a space discharge amplifier, a resistance networkincluded-in the control circuit of said amplifier, means for'passing arectified current through said'network-including a pair of controlchannels, the first channel producing a greater response for a givensignal volume than the second channel when the signal'volume is low, andmeans for limiting the response of said first channel so that the secondchannel predominates at high signal strengths.

'7. A system for producing sound from'a sound record comprising incombination with a space discharge amplifier, a resistance networkincluded in the grid return of said amplifier, means for passinga'rectified current through said network including a 'pair of controlchannels, the first channel producing a greater response for a givensignal volume than the second channel when the' signal volume is low;and means for strengths.

. 8. A system for producing sound from a sound record comprising incombination with a space discharge amplifier, a resistancenetworkincluded in the grid return of said amplifier, means for passing arectified current through said network including a pair of controlchannels, the first channel producing a greater response for a givensignal volume than the second-channel when the signals are low, meansfor limiting the response of said first channel so that the secondchannel predominates at high signal'st'rengths, and means to preventactuation of said first channel in response to signals of undesiredfrequencies.

9. A sy'stemfor producing sound from a sound record comprising incombination with a space discharge amplifier, a'resistance networkincluded'in the grid return of said amplifier, means for passing arectified current through said network including a pair of controlchannels, the first channel producing a greater response for a givensignal than the second channel for low signal strength {means forlimiting the response of said first channel so that the second channelpredominates at high signal strengths, and a filter network associatedwith said first channel to prevent actuation thereof in response tosignals of undesired frequencies.

' 10. A system' for producing sound from a sound record comprising incombination with a space discharge amplifier, means for controlling theamplification ratio thereof comprising a control channel;- said controlchannel being operative in response to sigi'ialsof desired frequenciesbut being inoperative in response to signals ofundesired-frequenciesunaccompanied by desired sigrecordcomprising incombination with a space discharge amplifier, a control channel forcontrolling the amplification ratio thereof, said control channel havingmeans associated therewith to prevent actuationthereof in response tohigh frequency signals, except when said'signals are of high intensityor are accompanied by signals of other frequencies. 1

12. A system for producing sound from a sound record, co'mprisinganelectron discharge amplifier, two control means'operable in accordancewith the strength of the input signal for varying the characteristics ofsaid amplifier, one of said control means operating to produce a largechange in amplification for predetermined changes in signal strength andhaving limiting means associated therewith to limit the effectivenessthereof to low signal strength, the other of said control meansproducing a'small change in amplification for said predetermined changesin signal strength, the arrangement being such that said first meanspredominates at small signal strength and said second means predominatesat large signal strength.

13. A system for producing sound from a sound record, comprising anelectron discharge amplifier having a variable gain ratio and controlmeans comprising a multiple stage amplifier operated in accordance withthe'indicated signal of said record, means controlled by one of saidstages for varying said amplification ratio as a predeterminedfunction'of the signal characteristics and means controlled by anotherof said stages for varying said ratio'as a different predeterminedfunction of the signal characteristics.

14. A system for producing sound from a sound record, comprising anelectron discharge amplifier having a variable gain ratio and controlmeans comprising a multiple stage amplifier operated in accordance withthe indicated signal of said record, means controlled by the first ofsaid stages for varying said amplification ratio as a predeterminedfunction of the signal and means controlled by the second of said stagesfor varying said ratio as a predetermined greater function of the signaland limited means for limiting the action of said second means wherebyit is effective only for small signals.

15. A system for producing sound from a sound record comprising anelectron discharge amplifier having a variable gain ratio and controlmeans comprising a two stage amplifier operated in accordance with theindicated signal strength of said record, means controlled by the firstof said stages for varying said amplification ratio as a predeterminedfunction of the signal strength and means controlled by the second ofsaid stages for varying said ratio as a predetermined greater functionof the signal strength, means for limiting the action of each of saidcontrol means to a predetermined signal strength and means for renderingone of said control means operative in response to a predetermined rangeof frequencies only. 1

16. A system for producing sound from a sound record, comprising a spacedischarge amplifier having variable amplification characteristics, meansfor varying said characteristics as a predetermined function of theindicated signal strength of said record when the signal strength issmall and means for varying said characteristics as a predeterminedsmaller function of the signal strength when said signal strength islarge, whereby a greater range of amplification is obtained at smallsignal strength than at large signal strength, and means for limitingthe action of said first mentioned control means in accordance with aselected band of frequencies only.

' 1'7. A system for producing sound from a sound record, comprising atwo stage amplifier, control means for controlling the amplificationcharacteristics of each of said stages, means whereby said control meansproduces a greater change in amplification of said first stage than ofsaid second stage, means whereby said controlmeans is caused to producea greater change in amplification factor when the input signal strengthis small than when the input signal strength is large, and means forlimiting the action of said control means for preventing overload ofsaid amplifiers.

18. A system for producing sound from a sound record, comprising atransducer varied by a plurality of similar automatic controls actuatedaccording to different functions of the transducer input signalcharacteristics.

19. In a sound reproducing system, a sound record, a pick-up deviceassociated therewith, an amplifier fed by said pick-up device, a loudspeaker fed by said amplifier, first and second control circuits fed bysaid pick-up device for determining the gain ratio of said amplifier,said first control circuit discriminating against weak, high frequencytone currents and having sensitive control action, but havingitslcontrol action with strong tone currents limited, said secondcontrol circuit having no substantial effect with weak signals butexerting a dominant control above the limiting point of said firstcontrol circuit.

20. A system for producing sound from a sound record comprising incombination with a space discharge amplifier, means for controlling theamplification ratio thereof comprising a control channel, said controlchannel being operative in response to signals of desired frequenciesand intensities but being inoperative in response to signals ofundesired frequencies and intensities unaccompanied by desired signals.

21. A system for producing sound from a sound record, comprising a spacedischarge amplifier having variable amplification characteristics, meansfor varying said characteristics as a predetermined function of theindicated low frequency signal strength of said record when said signalstrength is small and means for varying said characteristics as apredetermined smaller function of the total input signal strength whensaid signal strength is large, whereby a greater range of amplificationis obtained at small signal strength than at large signal strength.

22. A system for producing sound from a sound record, comprising atransducer the transmission characteristics of which are varied by aplurality of similar automatic controls, each actuated ac cording to thetransducer input signal strength, the variations effected by thedifferent controls constituting different functions of the change insignalstrength, at least one of said controls unresponsive to signals ofcertain predetermined frequencies.

23. In a system for reproducing sound from a sound record, an amplifieradapted normally to amplify energy derived from said record, means torender said amplifier inoperative when tones having the frequency andintensity characteristics of scratch tones are present and means torender said amplifier operative when tones of different frequencies orof different intensities are present.

24. The method of reproducing sound from a sound record by means of anamplifying system, which comprises varying the gain ratio of the systemin accordance with a plurality of different functions of the change instrength of the signal received from the sound record.

25. The'method of modifying the reproduction of a sound record whichcomprises varying the dynamics of the reproduction exclusively accordingto weak, low pitch notes for weak signals, and then according tostronger notes without regard to pitch for strong signals.

26. The method of modifying the reproduction of sound from a soundrecord by means of a reproducing system, which comprises rendering thereproducing system inoperative when weak, high frequency notes arepresent, and rendering the reproducing system operative when lowfrequency notes of any intensity or when strong high frequency notes arepresent, whereby the system is prevented from reproducing scratch tones.unaccompanied by signal tones.

27. The method of modifying the reproduction of sound from a soundrecord by means of an amplifier system, which comprises rendering theamplifier system inoperative when weak, high frequency notes alone arepresent, varying the gain ratio of the system as a function of theintensity of low frequency notes for weak sound energy value and varyingthe gain ratio of the system as a function of the intensity of notes ofall frequencies for stronger energy values.

28. In a system for use with a sound record, a pick-up device, anamplifier tube fed thereby and having a control electrode, a translatingdevice fed by said amplifier tube, a control circuit including a biasingimpedance for determining the bias on said control electrode and aplurality of limiting devices connected to said biasing impedance andoperating with different characteristics.

29. In a system for use with a sound record, a pick-up device, anamplifier tube fed thereby and having a control electrode, a translatingdevice 7 fed by said amplifier tube, a control circuit fed charge tubesconnected across said biasing resistance and operating at differentthreshold voltages.

30. In a system for use with a sound record, a pick-up device, a mainamplifier fed thereby, a translating device fed by said amplifier, acontrol channel also fed by said pick-up device and controlling the gainratio of said amplifier, said control channel comprising a plurality ofampli

